The present invention is directed to absorption cells. It particularly concerns off-axis cavity absorption cells.
In order to detect trace atmospheric chemical constituents in a gas sample by, for instance, infrared-absorption spectroscopy, the need to obtain measurable absorption often requires that a beam of light propagate a relatively long distance through the sample. To obtain this effect in a small space, a beam is often caused to reflect repeatedly between opposed mirrors.
Typically, an absorption cell is an elongated cylinder in which the mirrors are disposed at opposite ends, and light is coupled into and out of the cell through one or more holes in the mirrors, although some equivalent approach, such as the use of a small internal mirror, can also be employed.
A key factor that determines the cell's cross-sectional area, and thus its volume, is that the light beam should remain clear of the exit aperture before it has traversed the desired distance inside the cell. The spots at which the beam lands on each mirror always have a certain minimum size, which is imposed by the wave nature of light in the case of coherent light, such as that from a laser, or by the need to transmit a measurable flux from the source to the detector in the case of an incoherent source, such as a heated body. Given this constraint, the pattern that successive spots make on a mirror in a given configuration determines the cell size.
In the configuration described by J. White, "Long Optical Paths of Large Aperture," J. Opt. Soc. Am. vol. 32, pages 285-288 (1942), for instance, the spots occur in a single row. Most later implementations of that concept produce two parallel rows of spots, while the configuration described in Herriott et al., "Off-Axis Paths in Spherical Mirror Interferometers," Applied Optics vol. 3, pages 523-526 (1964), produces a circular or elliptical pattern. In all of these arrangements, the cell width increases relatively rapidly with the number of traverses that the beam makes of the cell's base length.
Herriott et al., "Folded Optical Delay Lines," Applied Optics vol. 4, pages 883-889 (1965) ("Herriott et al. II") describes a configuration that offers an important advantage over previous designs. In that configuration, the spots are spread over the surface in the mirror in two dimensions, so the required mirror diameter increases more slowly with the number of traverses. This configuration has not enjoyed wide acceptance, however, because its fabrication is difficult and costly.
The reason for this difficulty and cost is that the Herriott et al. II arrangement employs astigmatic mirrors, and for the beam to leave the cell at a point sufficiently close to the center of the (single) coupling aperture, the ratio of the mirrors' radii of curvature must be controlled to a very high accuracy. To realize a typical cell to provide a few hundred traverses in the mid-infrared region, for instance, requires that the relative error in that ratio be less than 0.01 percent. In most cases, the difficulty and cost of fabricating an aspheric mirror to that degree of accuracy are prohibitive.
To avoid that necessity, the astigmatism provided in Herriott et al. II was achieved by mechanically warping initially spherical mirrors. To obtain the greatest benefit from the astigmatic-mirror principle, however, the mirrors must have greater astigmatism than can be achieved as a practical matter by warping a spherical mirror. Even as a way to correct the curvature of a mirror with ground-in astigmatism, moreover, mechanical warping is disadvantageous. Specifically, stress relief in the warping elements (typically, springs) or in the mirror itself results in correction drift, and the temperature coefficients of the mirror's and warping elements ' elastic properties have to be closely controlled.